Liquid crystals are a well-known phenomenon. The first observation of the phenomenon was made in 1888 on cholesteryl benzoate, which melts at 145.degree. C. but to a cloudy liquid which does not become clear until 179.degree. C. Although apparently a liquid, the cloudy melt showed optical properties of a crystal and so the term "liquid crystal" was adopted.
There are three main groups of liquid crystals: smectic, nematic and cholesteric. Cholesteric liquid crystals have been used to map temperature patterns, as well as infra-red, microwave and ultrasonic fields through converting these energy forms into heat.
Until the discovery of liquid crystals, all matter had been commonly classified in three states: solid, liquid or gas, with increasing molecular disorder occurring as one passes from the solid to the gaseous state. The liquid crystalline state is in many respects a fourth state of matter intermediate in molecular ordering between a crystalline solid and an ordinary liquid. Hence it is more accurately named as the "mesomorphic" or intermediate state. However, the term "liquid crystal," first used by Lehmann in 1889, has remained in use because it describes so directly the properties of this intermediate state--the mechanical properties of liquids combined with the optical properties of crystals.
Between all molecules attractive and repulsive forces interact. In a crystalline solid the attractive forces between the molecules are strong enough to hold them packed into a regular three-dimensional geometrical array over large volumes. Hence crystals are said to show long range ordering of the molecules. One result of this ordering is that for certain crystals the physical properties, for example, the speed at which light travels through the crystal, will vary with the angle between the direction in which they are measuring and the axes of the crystal (which are defined by the way the molecules pack together). This variation of properties with direction of measurement is called anisotropic behavior.
In an ordinary liquid the cohesion between the molecules has been reduced to a point where they are free to move and so they can adopt a radom arrangement. There is still some degree of ordering as each molecule can be considered as being surrounded by a spherical shell of its neighbors. However, this spherical arrangement only holds for the short distances between neighboring molecules. Over longer distances the "short range" ordering of the molecules breaks down to form a random arrangement. Because of this long-range randomness the physical properties of a liquid are the same in whichever direction they are measured; that is, the liquid behaves isotropically.
For most organic compounds the transition from the crystalline state to the isotropic liquid occurs rapidly once the cohesive forces holding the molecules in a fixed arrangement in the crystal have been overcome. There is no stable intermediate level of molecular cohesion between the high level present in the crystal and the lower level present in the isotropic liquid. But some organic compounds, because of their rod-like crystalline shape and the particular attractive forces between the molecules, can have a stable intermediate level of molecular cohesion. It is these compounds which form "liquid crystals." In the liquid crystalline state the cohesion between the molecules has been reduced enough, compared to the crystalline solid, to allow a rearrangement of the molecules. Some freedom of movement is possible (thus the liquid properties) but not enough to allow completely random alignment of the rod-like molecules (thus the anistropic crystalline behavior). Eventually, with a further increase of, for example, temperature, the cohesive forces in the liquid crystalline state (mesophase) are overcome and an ordinary isotropic liquid is formed.
The three main subdivisions or mesophases of the liquid crystalline state (smectic, nematic and cholesteric), all have some basic properties in common, such as birefringence, but they differ in their molecular structure and other properties.
(a) Smectic mesophase: in the smectic mesophase, the molecules are arranged in "raft like" layers, with their axes parallel, either normal (that is, at right angles) to the plane of the layer, or tilted. The molecular packing within the layer can be either regular or random.
(b) Nematic mesophase: in the nematic phase the long axes of the molecules retain a parallel alignment but, in contrast to the smectic phase, there is no separation into layers so that otherwise their positional arrangement is random. Thus this type of liquid crystal is that much closer to an ordinary isotropic liquid than the smectic phase.
(c) Cholesteric mesophase: strictly speaking, the cholesteric phase is a "twisted" form of the nematic phase. There is no layering and the positional arrangement of the molecules is random, but the direction in which the parallel molecules are aligned twists round as one passes through the cholesteric phase. This twisting forms a screw-like, helical arrangement of the molecules in the cholesteric phase, like the steps of a spiral staircase. Because of this helical structure a property very important to the optical behavior of the cholesteric phase is produced; that is, a periodicity corresponding to the pitch of the helix (the distance between areas where the molecules are pointing in the same direction) which is roughly equal to the wavelength of visible light.
The most impressive and striking optical property of the cholesteric liquid crystal is that under the right conditions it displays a vivid color when illuminated with white light. Basically, the cholesteric liquid crystal is behaving like a mirror reflecting light falling onto it, but it is no ordinary mirror since it picks out and reflects only light of a certain wavelength, a certain color, which corresponds to the pitch of the helix. By regarding the molecular arrangement as a set of thin birefringent plates (the molecular layers of roughly parallel alignment) stacked in a helix one is able to analyse the rather unusual optical properties of the phase. Birefringence is the property whereby light passing through a crystal experiences double refraction because it can have two different velocities as a result of the anisotropy of the crystal. Light rays entering the stack pass from plate to plate undergo a small sideways bending at each transition because of the birefringent refractive properties of each plate. This sideways twisting of the light rays causes the light to become circularly polarised as it travels further down through more plates so that it ends up spiralling along the axis of the helix. A further result of the birefringence of the plates is that the light separates into a fast and a slow wave between which a phase shift will occur (light waves from the fast component will get out of step with light waves from the slow component). At certain points along the helix the phase shift between the two wave sets up a standing wave reflecting circularly polarised light back out of the cholesteric liquid crystal. The standing wave will have a wavelength equal to the pitch of the helix and it is this color which is seen. For light of other wavelengths the conditions for setting up a standing wave are not present and so this light will continue on through the cholesteric liquid crystal unreflected. Thus if white light falls on the crystal one color will come back out as a reflected wavelength while other colors will be transmitted onwards. If the light that is transmitted is finally absorbed into a dark background only the reflected wavelength is seen, as a pure iridescent color.
Described above is a characteristic method of using a cholesteric liquid crystal, that is, as a thin film of material deposited on a light-absorbing black background. Subjected to the appropriate temperature or other stimuli the film reflects the vivid colors of the rainbow. For example, a sheet of cholesteric liquid crystal sensitive in the appropriate temperature range (25.degree. C. to 31.degree. C.) will change color rapidly in response to body warmth if it is handled. The color observed depends on the pitch distance of the helix in the crystal and this pitch can be easily altered. The distance between the molecular layers in the crystal which determines the final pitch distance depends on a balance of weak intermolecular attractions and repulsions. In response to a change in this balance by a stimulus, such as a temperature change, shear force and presence of other chemicals, a color change will be seen. It is this direct, visible response to stimuli which gives cholesteric liquid crystals their important versatility as detector systems. The stimuli can either interact directly with the balance of molecular forces in the crystals or can be converted into a stimulus which does interact.